A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to impart a beam of radiation with a pattern in its cross-section, the pattern corresponding to a circuit pattern to be formed on an individual layer of the IC. This pattern can be imaged or transferred onto a target portion (e.g. comprising part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an image of the entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the semiconductor manufacturing industry there is increasing demand for ever-smaller features and increased density of features. The desired critical dimension (CD) of a smallest feature to be printed lithographically is rapidly decreasing and is becoming very close to the theoretical resolution limit of state-of-the-art exposure tools such as steppers and scanners as described above. Conventional technologies aimed at enhancing resolution and minimizing printable CD include reducing the wavelength of the exposure radiation, increasing the numerical aperture (NA) of the projection system of the lithographic apparatus, and/or including features in a patterning device pattern smaller than the resolution limit of the exposure tool so that they will not print on the substrate, but so that they will produce diffraction effects which can improve contrast and sharpen fine features.
When such conventional resolution enhancement techniques are applied to a lithographic printing process wherein a desired pattern to be printed is repetitive along two different directions, the repetitiveness characterized by two corresponding different pitches, a size-error of a feature as printed may be beyond tolerance. This may be due to a problem known as the optical proximity effect. This is caused by the inherent difference in the diffraction pattern for isolated features as compared to dense features. Dense features may include nested patterns and closely spaced periodic features. The optical proximity effect may, for example, lead to a difference in CD when dense and more isolated contact holes or lines are printed at the same time.
The optical proximity effect also depends on the illumination setting used. Originally, so-called conventional illumination modes have been used which have a disc-like intensity distribution of the illumination radiation at the pupil of the projection system. However, with the trend to imaging smaller features, off-axis illumination modes have become standard in order to improve the process window, i.e. the exposure latitude in combination with depth of focus, for small features. However, the optical proximity effect becomes worse for off-axis illumination modes, such as annular illumination.